Select the group you would like to manage the logo for, or click Edit Site-wide Skinning Defaults to apply the change throughout the site

Expand the section called Header

Under Logo Image, click Edit All. Enter the image file path as well as the height and width dimensions (in pixels). You may also need to adjust the margin depending on how tall the image is. Click Save Changes when you are done.

Scroll down further to the Logo Show Text sub-section and set the display: field to none. This will hide the Site Title and Tagline text.